Is it traffic-related air pollution or road traffic noise, or both? Key questions not yet settled!

Abstract

There is substantial evidence for the association of longterm exposure to both road traffic noise and to ambient and traffic-related air pollution on cardiovascular diseases (CVDs) (Brook et al. 2010; van Kempen and Babisch 2012). As traffic is the main common source of noise and air pollution, the question remains whether there is a mutual confounding effect of these stressors on common cardiovascular outcomes (Brook et al. 2010; Babisch 2011). Identifying whether air pollution and/or noise drive this association is relevant to implement adequate abatement policies. In this issue, Tetreault et al. (2013) provide the first systematic review that assesses whether the cardiovascular effects of long-term exposure to road traffic noise or to air pollution are mutually confounded based on the existing literature. Interestingly, this review comes to the conclusion that there is not much confounding (i.e. a change below 10 % in effect estimates), although this confounding is higher in some of the 9 studies that met the inclusion criteria. Although this finding could be interpreted as that there is no need to adjust for traffic-related air pollution or noise in studies, the authors acknowledge that, due to the scarce literature, this review cannot draw a final conclusion regarding the degree of mutual confounding effects of these environmental factors on their long-term association with any cardiovascular outcome. Moreover, while a low confounding effect may well be true in several settings, findings may not be generalized to all study populations. Indeed, many factors may contribute to the confounding, such as the health outcome assessed, the study design, the exposure assessment, the population and urban characteristics, and the sample size. The review points to several key questions. First, conceptually, the degree of confounding will depend on the magnitude of the relationships between the exposures (i.e. air pollution and noise) and between the exposures and the outcome. In turn, the confounding may change across study areas given that the spatial correlation between trafficrelated air pollution and road traffic noise can vary with urban features (e.g. Allen et al. 2009; Foraster et al. 2011). However, Tetreault et al. (2013) do not see an influence of the correlation on the confounding effects across studies. Although authors propose that this agrees with a low confounding, it may indeed relate to the fact that not all studies observed associations between the main exposure (e.g. air pollution) and the outcome, and the same may apply to the confounding factor (e.g. noise). In any case, the observation by Tetreault et al. (2013) would call for the assessment of the confounding effect irrespective of the correlation observed in each study area. Second, different noise and air pollution indicators were used in the literature, which may contribute to the heterogeneity in the confounding effect across studies. Besides, the air pollution indicators were not always specific of the traffic-related emissions, which would lead to an inadequate control for the potential confounding effect. Third, the authors acknowledge that not all studies provided information on the quality of the exposure assessment, thus preventing the interpretation of the quality of the adjustments. In addition, it should be noted that the final treatment of the exposure variable (e.g. categorization, This Editorial is part of the special issue: ‘‘Environment and Health Reviews’’.